RESTRICTIVE SMTP AUTH:- Enable "SmtpConfig" "mail-auth" in your server.tab- Enable SMTP Authentication on your client- This makes your system private to your domain users only other SMTP's cannot send mail to your server, because they cannot authenticate.- If you send mail to other SMTP servers, they might blacklist you as a private spam server.- Enabling RESTRICTIVE SMTP AUTH is generally a bad idea.

OPTIONAL SMTP AUTH:- Disable "SmtpConfig" "mail-auth" in your server.tab- Clear smtprelay.tab or limit it to your LAN, only.- Enable SMTP Authentication on your client- Even when you have disabled "mail-auth" in your server.tab, XMail accepts client requests for SMTP Auth.- This won't get your server marked as a spam server ... - ... and other SMTP's can deliver mail to your system without authentication.

So if you disable POP_BEFORE_SMTP (with "EnableAuthSMTP-POP3" "0" in your server.tab) and use OPTIONAL SMTP AUTH, you deny relaying and force your own users to authenticate.